Blocks Support: Add BlockDocument HTTP client + struct model

[parkedwards]

https://www.notion.so/prefect/Design-Block-support-in-the-Terraform-Provider-ae2ea25a18864de9b51b8aa06af9486b?pvs=4

A block_document is the actual API object that holds the user's value

Things we'll probably need:

• api.BlockDocument (struct) - in internal/api (the API response object0
• api.BlockDocumentCreateRequest
• api.BlockDocumentUpdateRequest
• BlockDocumentClient interface{}- in internal/client
• func (c BlockSchemaClient) Get(ctx context.Context, ID uuid.UUID) (api.BlockDocument, error) - in internal/client
• func (c BlockSchemaClient) Create(ctx context.Context, payload api.BlockDocumentCreateRequest) (*api.BlockDocument, error) - in internal/client
• func (c BlockSchemaClient) Update(ctx context.Context, ID uuid.UUID, payload apiBlockDocumentUpdateRequest) (*api.BlockDocument, error) - in internal/client
• func (c *BlockSchemaClient) Delete(ctx context.Context, ID uuid.UUID) (error) - in internal/client

note that we can also include a ?include_secrets=true query param, so that we can always hydrate the TF state with the actual value